1. Technical Field
The invention relates to a stator design for an electric machine capable of acting either as a motor or as a generator.
2. Background Art
It is known practice in the design of a conventional, brushless, multiple-phase motor-generator to use a rotor with permanent magnets arranged on its periphery, and a stator with a plurality of stator poles surrounding the permanent magnets. A magnetic flux flow pattern is developed when stator coils wound on the stator are energized with a multi-phase sinusoidal voltage.
The output torque of a multiple-phase electric machine comprises a uniform torque component and a varying torque component. The varying torque component is developed because of harmonic magnetic flux distributions in the stator and the rotor. The total torque output of the electric machine is the combination of the uniform component and the varying component, which results in a phenomenon called torque ripple. This torque ripple causes driveline speed oscillations, which can result in vehicle vibration and noise as the electric machine vibrations resonate in the vehicle body and chassis structure. The phenomenon of torque ripple is particularly significant at low vehicle speeds.
U.S. Pat. No. 6,867,524, which is assigned to the assignee of the present invention, describes one solution to the problem of torque ripple in electric machines. That solution involves forming the rotor of an electric machine into multiple segments. These segments are comprised of stacked permeable metal laminations with peripheral openings for receiving permanent magnets. As the magnet poles move across the stator poles, the magnetic flux field of the magnets interacts with the magnetic flux field of stator electrical windings that are energized by a multiple-phase voltage. The segments are skewed about the geometric axis of the rotor, one with respect to the other, so that the torque ripple created by a pole of each segment will tend to cancel the effect of the torque ripple of an adjacent pole so that the combined separate torque ripple components will be attenuated.